In colder climates where fluid carrying apparatus may be subjected to sub-zero temperatures, reliability of such equipment may be compromised by freeze damage. Under cold temperature conditions, working fluids comprised of aqueous liquids may freeze and, consequently, expand. If the working fluid is confined within rigid fluid passage walls, its expansion during freezing will be resisted. As a result, when freezing occurs, pressure increases within the fluid passage, which, depending on the rigidity, could lead to failure and rupture of the structure of the fluid passage after one or more freezing cycles.
Such reliability concerns exist with solar thermal collectors. The conversion of solar radiation to heat for building applications is achieved by circulating a working fluid, typically an aqueous liquid, through an arrangement of channels or like conduits or tubes that are typically in contact with a highly thermally conductive sheet whose selectively coated surface is exposed to the incident solar radiation. The heated fluid is then distributed to thermal storage and to building applications, such as space heating and domestic hot water heating. The actual solar radiation conversion device is a solar thermal collector.
Presently, three types of solar thermal collectors exist in building applications that embody the combination of a selectively coated sheet, or absorber sheet, and fluid-carrying channels or tubes. They are the flat plate collector, the heat pipe collector, and the evacuated tubular collector. Each type of collector can be divided into two functional zones: the collector absorber unit and the absorber unit encasement. The collector absorber unit consists of the assembly of the fluid carrying apparatus and the absorber sheet. Such working fluid carrying apparatus includes fluid passages defined by rigid walls for facilitating the transportation of the fluid through the collector absorber unit.
The prior art features several attempts to improve the ability of the collector absorber unit to endure conditions of fluid freezing for over 20 years of operation. Several means of dealing with the freezing of the working fluid have been proposed over the years by the solar industry, and by other fluid handling sectors as well.
In one method, water is removed from zones in the collecting apparatus where freezing conditions are expected. This strategy has been incorporated into the operation of drain-back and drain-down solar thermal systems. A control system signal initiates the removal of the fluid from the fluid passage that is exposed to the freezing conditions. In this case, reliability of the control system becomes critical. If the working fluid fails to be removed, then nothing protects the fluid carrying apparatus from freezing damage.
In accordance with another method, fluids with freezing points that are lower than temperatures expected during operation are used. In general, such fluids are expensive, offer poor thermal properties, and involve high fluid handling costs. In all cases, they fail to provide significant gains in cost-effectiveness and do not justify their contribution to the prevention of freezing. Prime examples of this method are the circulation of silicone oils or air in the solar thermal collector loop. Silicon oils are ol high cost. They have thermal capacities which are lower than water. To match the thermal performance of the water, silicone oils must be circulated at high mass flows that cannot be justified because of the resultant increase in fluid handling costs, such as pump costs. Moreover, silicone oil is challenging to contain, thus necessitating additional cost-generating measures. Air has a very low thermal capacity and low thermal conductivity and therefore, is only appropriate for space heating systems, which limits its scope of application.
A further means of preventing freeze damage is to inject chemical additives to lower the freezing point of the working fluid. The chemical stability of such mixtures is questionable over long operating periods, thus adding maintenance costs and creating new reliability concerns. Moreover, this method must conform with local regulations relating to domestic water contamination, thus incurring additional costs. A conventional application of this method in solar thermal systems is the addition of propylene glycol to water. Historically, this mixture has been known to break down at collector stagnation temperatures while in the presence of oxygen. Without proper maintenance the propylene glycol mixture forms clumps and becomes corrosive over time, which results in a loss of performance and capacity to protect from freezing.
Another means disclosed in the prior art is the use of a flexible and resilient insert inside the fluid channels of the solar thermal collector. Examples of such inserts are disclosed in U.S. Pat. Nos. 5,579,828; 4,227,512; 4,321,908; and 3,989,032. The insert can accommodate the expansion that the working fluid experiences upon freezing and thus protects the fluid carrying apparatus from rupturing. Although this design is very effective in freeze protection, it creates additional manufacturing costs (e.g. outfitting the fluid carrying apparatus with the insert). The operation of the insert is also plagued with reliability and durability concerns. Typically, the insert consists of a skin or sheath filled with a gas at a pressure higher than the working fluid pressure. The sheath or skin may be comprised of thin metal sheets, plastics, or elastomeric tubes. For a collector life that is expected to extend beyond the 20 year mark, flexible metallic inserts are unreliable because they will typically fail as a result of material fatigue, thermal aging, corrosion, or a combination thereof. Increasing the metal thickness to delay metal deterioration, due to corrosion or thermal aging, only increases the costs and curtails flexibility and capacity to accommodate the expansion of a freezing fluid. Over similar operating periods, plastic thin-walled inserts may get saturated with the working fluid since plastics are permeable to most fluids typically used in heat transfer applications. This renders the insert ineffective for freeze protection. In the case of solar thermal collectors, increasing the skin thickness to decrease permeability will unjustifiably increase the material costs since expensive high temperature plastics, such as Teflon.TM., are required to withstand stagnation conditions (which can exceed 200.degree. C.). Similar material costs and saturation issues apply to elastomeric inserts. Furthermore, thin walled pressurized designs, whether made of metal or plastic, are not robust because they are vulnerable to skin puncture, which subsequently renders the insert ineffective upon depressurization.
Other attempts in dealing with the freeze problem have included using conduits made with flexible and resilient walls to accommodate fluid expansion upon freezing, such as that disclosed in U.S. Pat. No. 4,299,200. The walls of such conduits are made of plastics, elastomers, or metals, and therefore are similar to the materials used within the above-described inserts. As a result, this particular solution does not escape the same issues of robustness, material costs, metal fatigue, and corrosion which arise in association with the use of the insert. In addition, the use of plastics or elastomers as channel walls may impede performance of solar thermal collectors, or any heat exchanger application where this solution is incorporated because of the low thermal conductivity characteristics associated with such materials.
In yet another means of freeze protection, a controlled heat source is employed in the regions where freezing of the working fluid is anticipated. As in the first-mentioned prior art attempt at dealing with this problem, the reliability issue is not solved, but merely shifted to the operation and control of the heat source. Further, with respect to solar thermal applications, this solution adds new components and thus additional costs.